1. Field of the Invention
The present invention relates to an optical-use adhesive film, and in particular to an adhesive film used for, for example, a base film for a prism lens sheet in an LCD, a base film for a hardcoat-processed film, a base film for an AR (anti-reflector) film or a protective film in a CRT.
2. Description of the Related Art
In general, an optical-use adhesive film includes a substrate film and an adhesive layer provided on a surface of the substrate film. The substrate film is formed of, for example, a biaxially oriented polyester film.
Biaxially oriented polyester films are in wide use as various optical-use films due to the superior transparency, size stability and anti-chemical resistance thereof.
In particular, base films for prism lens sheets used in LCDs, base films for hardcoat-processed films, base films for AR films and protective films used in CRTs are required to have a superior strength and size stability. Accordingly, films having a relatively large thickness of about 100 xcexcm or more are preferably used for the above-mentioned optical-use films. Such optical-use films are required to have a superior transparency and also to have a superior adhesiveness suitable for prism lens processing, hardcoat processing and AR processing. It is demanded that microscopic foreign substances contained in such films are in a minimum possible amount since such foreign substances are recognized as optical defects.
It is generally known that a polyester film has an inferior adhesiveness with, for example, prism lenses or hardcoat layers formed of an acrylate or the like. In order to provide the film with a satisfactory slip and ease in handling, it is common to mix inactive particles in a polyester film to form an unevenness at a surface of the film. However, the inactive particles mixed in the polyester film deteriorate the transparency of the film and thus is undesirable when the polyester film is used for optical uses.
Presence of foreign substance particles in an optical-use polyester polymer film is undesirable for the following reasons. When a substrate film of the optical-use film is formed of the polyester polymer, the alignment of the polyester molecules is disturbed around the foreign substances. Thus, an optical distortion tends to be generated. Due to the optical distortion, the foreign substance particles are recognized as optical defects much larger than the actual size thereof, which significantly spoils the quality of the film. For example, even a foreign substance particle having a size of about 20 xcexcm is recognized as an optical defect having a size of about 50 xcexcm or more, and in an excessive case, 100 xcexcm or more. It is desirable that, in order to provide a highly transparent film, the inactive particles for providing the film with a satisfactory slip are not mixed in the film or are mixed in a very small amount such that the transparency of the film is not spoiled. However, as the amount of the inactive particles is smaller and thus the transparency of the film is higher, the optical defects caused by the microscopic foreign substance particles tend to be more conspicuous. As the film is made thicker, the amount of the foreign substance particles in a unit area of the film tends to be larger, which makes more serious the problem of the optical defects.
Reducing the amount of the inactive particles mixed in the film to a very small amount or zero in order to raise the transparency of the film has another problem in that the film does not have a sufficient slip and is difficult to handle. Such an insufficient slip can be avoided by mixing slip-providing particles in an adhesive layer of the film. Such slip-providing particles need to have an average size which is smaller than the wavelength of the visible light in order to maintain the transparency of the film. However, such microscopic particles tend to aggregate together to form a rough and large aggregation. Such large aggregations in the adhesive layer are also recognized as optical defects as well as other foreign substance particles also contained in the adhesive layer.
An optical-use adhesive film needs to have a thermal shrinkage which is small and uniform in all directions. For example, an optical-use adhesive film used for touch panels is provided with a transparent conductive layer laminated on one surface thereof and a hardcoat layer laminated on the other surface thereof. The surface on which the hardcoat layer is to be laminated is, for example, annealed as post-processing. The annealing is performed at a temperature of about 130xc2x0 C. to about 150xc2x0 C. Unless, the thermal shrinkage of the film at about 150xc2x0 C. is small and uniform in all directions, the film is significantly curled by the annealing. As a result, the film may become stuck in a drying apparatus, or may not provide a satisfactory flatness when used in a touch panel unit. A film as thick as about 100 xcexcm or more has a slower heat transfer and thus is more satisfactory in thermal shrinkage than a thinner film. However, regardless of the thickness, a rolled film coming out from a roll mill through a slit at an end of the mill has a different angle of orientation axis from a portion of the film running at a center of the mill, due to the so-called xe2x80x9cBowing phenomenonxe2x80x9d. For this reason, the film tends to have an anisotropic thermal shrinkage, which results in curling.
An optical-use adhesive film according to the present invention includes a substrate film formed of a biaxially oriented polyester film; and a polymeric adhesive layer provided on at least one surface of the substrate film. The optical-use adhesive film has a haze of 1.0% or less. and the substrate film contains foreign substance particles having a maximum size of about 20 xcexcm or more at a ratio of about 10/m2 or less.
In one embodiment of the invention, the substrate film has a thickness of about 100 xcexcm or more.
In another embodiment of the invention, the substrate film contains the foreign substance particles having a maximum size of about 20 xcexcm or more at a ratio of about 10/m2 or less in and below an area of the substrate film, the area having a protrusion having a height of about 1 xcexcm or more and a recess bordered on the protrusion and extending a horizontal distance of up to about 100 xcexcm from the border with the protrusion, the recess having a depth of about 0.5 xcexcm or more. In this specification, such an area is referred to as a xe2x80x9clens areaxe2x80x9d. For simplicity, the expression xe2x80x9cin the lens areaxe2x80x9d refers to xe2x80x9cin and below the lens areaxe2x80x9d unless otherwise specified.
In still another embodiment of the invention, the polymeric adhesive layer contains a polyester copolymer and a polyurethane resin.
In yet still embodiment of the invention, the polymeric adhesive layer contains slip-providing particles.
In yet still embodiment of the invention, the adhesive layer contains at least either one of rough and large aggregations and foreign substance particles having a maximum size of about 100 xcexcm at a ratio of about 3/m2 or less.
In yet still embodiment of the invention, the optical-use adhesive film has a maximum thermal shrinking ratio of about 3% or less and maximum thermal shrinking ratio/minimum thermal shrinking ratio of about 1 to about 10 after being treated at about 150xc2x0 C. for about 30 minutes.
Thus, the invention described herein makes possible the advantages of providing an optical-use adhesive film which is superior in transparency, adhesiveness, thermal shrinkage, and optical defects.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.